IMHO As China is downwind from North Korea, they will do everything they can to prevent a nuclear exchange on the Korean Peninsula.
Upwind. China is most definitely upwind. Japan is downwind. Winds tend to flow from the West to the East. Has to do with the earths rotation and air density when heated by the sun and relative air pressures. Ever remember wind coming directly from the East. Only seen it a few times. Spinning hurricane remnants and other weird weather anomalies.
Prevailing winds are easterly, not the other way around. Japan would be at a greater threat.